Coatings having high solar absorptance to infrared emittance ratios

ABSTRACT

A FORMULATION PROVIDING COATING HAVING HIGH SOLAR ABSORPTANCE TO INFRARED EMITTANCE RATIOS COMPRISES OXIDIZED COPPER FLAKES, A POLYMERIC BINDER AND A SOLVENT FOR THE BINDER. THE OXIDIZED COPPER FLAKES MAY BE PREPARED BY HEATING IN AN OXIDIZED COPPER FLAKES MAY BE PREPARED BY INCLUSION IN THE COATING FORMULATION. OPTIONALLY, COPPER FLAKES MAY BE DISPERSED IN THE COATING FORMULATION FOLLOWED BY APPLYING THE FORMULATION TO THE SUBSTRATE TO BE COATED FOLLOWED BY CURING OF THE POLYMERIC BINDER. THEN THE COPPER FLAKES EMBEDDED IN THE BINDER MAY VE OXIDIZED BY HEATING THE COATED SUBSTRATE IN AN OXYGEN CONTAINING ATMOSPHERE. THE FORMULATION IS PARTICULARLY USEFUL FOR COATING SURFACES OF MISSILE AND SPACECRAFT STRUCTURES WHERE THERMAL BLANCE IS OF PRIME IMPORTANCE. IN GENERAL, THE COATED SURFACES HAVE A SOLAR ABSORPTANCE TO INFRARED EMITTANCE RTIO RANGING FROM 1.80 TO 2.60.

33%,??? Patented May M, 1974 No Drawing. Continuation-impart of abandoned application Ser. No. 28,138, Apr. 13, 1970. This application Mar. 23, 1972, Ser. No. 237,494 The portion of the term of the patent subsequent to Apr. 4, 1989, has been disclaimed Int. Cl. B44d /06; G02b 5/20 US. Cl. 117-333 3 Claims ABSTRACT OF THE DISCLOSURE A formulation providing coatings having high solar absorptance to infra r ed emittance ratios comprises oxidized copper flakes, a polymeric binder and a solvent for the binder. The oxidized copper flakes may be prepared by heating in an oxygen containing atmosphere prior to the inclusion in the coating formulation. Optionally, copper flakes may be dispersed in the coating formulation followed by applying the formulation to the substrate to be coated followed by curing of the polymeric binder. Then the copper flakes embedded in the binder may be oxidized by heating the coated substrate in an oxygen containing atmosphere. The formulation is particularly useful for coating surfaces of missile and spacecraft structures where thermal balance is of prime importance. In general, the coated surfaces have a solar absorptance to infrared emittance ratio ranging from 1.80 to 2.60.

This application is a continuation-in-part of pending US. patent application Ser. No. 28,138, filed on Apr. 13, 1970, and now abandoned.

This invention relates to coating compgs' having igh solar absgrplagce.to.infrdfdemittqlli os. In one aspect ifrelates to compositions ili'at are p iiticularly adapted for coating surfaces of missile and spacecraft structures. In another aspect it relates to a process for coating substrates, such as surfaces of missiles and spacecraft structures, in order to control thermal balance,

Successful thermal balance of a spacecraft depends in part upon the optical properties of the surface coatings and the behavior of these properties in the space environment. The thermophysical properties that are of interest to thermal designers are (1) solar absorptance (a the fraction of incident solar energy which a surface absorbs, and (2) the infrared emittance (e the fraction of heat that a surface radiates compared to that which a blackbody would radiate at the same temperature.

One type of thermal control material available to spacecraft designers is called the solar absorber surface. Such surfaces absorb solar energy (oz O.4) while emitting only a small percentage of the infrared energy (e 0'.25 Thus, their (XS/61R ratios are greater than unity which makes it possible to maintain surfaces at high temperatures while the craft is traveling through space. This is of great importance since it is necessary to maintain certain types of equipment associated with a spacecraft at a high temperature in order to ensure satisfactory operation.

In the past, solar absorber surfaces have been obtained by the use of polished metals, chemical conversion coatings, and thin film techniques, such as vacuum evaporation, sputtering, electron beam, and thermal decomposition of organometallic compounds. Another method involves pattern painting where the or /e ratio can be varied by adjusting the percent of surface coverage by one coating system with a given set of optical properties over another coating system with vastly different optical mm on Ila-manta properties. These techniques to obtain high en /e ratios are limited from the standpoints of ease of application, especially to complex geometrical surfaces, maintenance and repairability, cost, and weight contribution.

The limitations listed in the preceding paragraph could be overcome or alleviated by the development of a suitable coating that could be applied as a paint. Paints con taining a binder filled with aluminum powder having an (ZS/61R ratio of 1.0 to 1.2 have been used for several years for satellite applications. The principal problems associated with metal pigmented paints with respect to obtaining higher en /e ratios have been their low a, values (0.25-0.40) and the fact that the polymeric binders absorb infrared energy, thereby causing high emittance values (040+). Comercially available paints containing metal particles other than aluminum, such as copper or bronze, may have high solar absorptance values, but their formulation is such that they do not have the necessary low emittance.

It is an object of this invention, therefore, to provide a paint formulation from which a coating having a high (Z /Em ratio can be prepared.

A further object of the invention is to provide a process for forming a coating on a spacecraft or missile surface that has a high (I /61R ratio.

Another object of the invention is to provide low cost surface coatings for missiles and spacecraft that can be readily repaired and maintained.

The present invention resides in a coating formulation from which coatings having high tX /E ratios can be obtained. Broadly speaking, the formulation comprises a dispersion of oxidized copper flakes in a solution of a polymeric binder. The formulation can be conveniently prepared by dissolving the polymeric binder in a solvent for the binder after which the oxidized copper flakes are added to the solution. Prior to use the solution is stirred to ensure that the flakes are dispersed throughout the solution. A preferred mixing method involves adding the materials to a cylindrical glass container, closing the container with a Teflon cap, and then placing the container on a two-roll roll mill. Rotation of the rolls causes the container to revolve, thereby dispersing the flakes in the solution.

As previously mentioned, the formulation of this invention provides coatings having high (l /61 ratios. In general, the ratios vary from about 1.80 to 2.60 depending upon several factors, including oxidation conditions, method of oxidation, the binder used, and the weight ratio of V oxidized copper flakes to hinder. In particular, the absorptance values are greatly increased as a result of theuse of oxidized flakes whereas the emittance values are only slightly increased. The ability of a spacecraft to meet its design mission is inseparably linked with maintaining the temperature of its. contents within the relatively nar-= row design limits where they operate best. The controlling element which determines the temperature of the vehicle is the ability of its external surface to exchange energy with its environment. The formulation of the present in vention with its capability to form coatings having high en /6 ratio values furnishes an important supplement for accomplishing the required overall temperature balance in spacecraft.

The copper flakes that are oxidized prior to use in preparing the paint formulation are available from commercial sources. They generally vary somewhat in size and are usually irregular in shape. It is preferred to use flakes whose diameter or width is less than 150 microns, preferably in the range of 40 to microns. The thickness of the flakes, of course, also varies, but it is usually in the range of 0.25 to 2.5 microns. The flakes are oxidized by heating them at an elevated temperature in the presence of an oxygen-containing atmosphere, for a time suflicient to ensure that an oxide layer is formed on their surfaces. The oxidation is preferably carried out in air at a temperature in the range of about 100 to 300 F. and for a period of about 5 to 60 minutes. In any event the oxidation is continued until the surfaces of the flakes are dark in color.

In general, any of the well-known resins, including those used in the paint art, can be employed as the polymeric binder in preparing the formulation. Examples of suitable binders include phenolic resins, such as phenolformaldehyde, resorcinol-forrnaldehyde and phenol-furfural resins; polyorganosiloxanes, such as polymethylsiloxane and polyphenylsiloxane; acrylic resins, such as polyacrylonitrile and polymers of methyl methacrylate; alkyd resins, such as those prepared from phthalic anhydride, maleic anhydride or fumaric acid, and a polyhydric alcohol, such as glycerol, pentaerythritol or sorbitol; nitrocellulose plastics; terephthalate polyesters, such as polyethylene glycol terephthalate; vinyl polymers, such as polyvinyl chloride, polyvinyl acetate, polyvinyl aldehyde, and copolymers of vinyl chloride and vinyl acetate; and the like. It is to be understood that mixtures of resins can be utilized as the polymeric binder.

A number of solvents can be used in preparing the formulation of this invention. The solvent selected will depend upon the particular resin used as the binder, and such a selection is well within the skill of the art. Examples of suitable solvents include aliphatic hydrocarbons, such as pentane, hexane and isooctane; cycloaliphatic hydrocarbons, such as cyclopentane and cyclohexane; aromatic hydrocarbons, such as benzene, toluene and xylenes; alcohols, such as methanol, ethanol and butanol; ketones, such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; acetates, such as vinyl acetate, Cellosolve acetate; and the like.

The amount of each component in the formulation can vary within rather broad ranges. Thus, the formulation can contain on a weight basis 3 to 80 percent oxidized copper flakes, 1 to 20 percent polymeric binder and 19 to 95 percent solvent. To obtain the highest en /6 ratios, it has been found that the amount by weight of the oxidized copper flakes must be greater than the amount by weight of the binder, preferably from 5 to 8 times greater. However, it is to be understood that good results can be obtained with lower proportions or loadings of the flakes. By using the higher proportions of flake material as compared to the binder, a minimum of the binder in the coating prepared from the formulation is exposed to infrared radiation. As a result the binder is substantially prevented from absorbing infrared energy which would otherwise cause high emittance values. As compared to unoxidized copper flakes, it is possible to obtain much higher loadings with the oxidized flakes. This results in superior film integrity and adhesion for coatings prepared from the formulations of thisinvention.

The formulation can be applied to the surface or portion of the surface of the spacecraft by brushing or spraying. However, it is preferred to spray the formulation since a more even coating is obtained and locations dilficult to reach are more readily coated. Curing of the coating can be accomplished by merely allowing the coated specimen to remain at room temperature for a period of time suflicient for the solvent to evaporate and the coating to harden. The period can vary within rather wide limits, for example, from about 8 to 48 hours and longer. The time necessary to cure the coating can be appreciably shortened by carrying out the procedure at an elevated temperature, for example, from about 50 to 150 C. It is also within the scope of the invention to add a curing agent to the formulation. The particular curing agent used will depend upon the polymeric binder employed in the formulation. For example, organotin compounds, such as tin octoate, can be utilized as curing ag h a p y g os lqxa s s used as the binder- Curing agents suitable for use with the various resins are well known to those skilled in the art. However, compounds containing active hydrogen should not be used as the curing agent since such compounds may reduce the copper oxide. For example, epoxy resins often employ amine curing agents so that such a system should be avoided in the practice of the present invention.

When the formulation is sprayed on the surface or substrate to be coated, the oxidized copper flakes float on top of the thin layer of the solution. As the solvent evaporates, the flakes become embedded in the binder in contiguous relationship with one another, thereby forming a substantially continuous layer of flakes. A very thin film of the polymeric binder covers the flakes.

According to the foregoing description of the preparation of the formulation, the copper flakes are oxidized prior to being dispersed in the binder solution. In another embodiment of the invention, a dispersion of unoxidized copper flakes in a binder solution is prepared. A substrate is then spray coated with the dispersion after which the resulting coating is cured. The coating is then subjected to oxidizing conditions as described hereinbefore with the result that the copper flakes are oxidized while embedded in the cured coating. It has been found that maximum a /e ratios are obtained when the coating is prepared in this manner.

A more complete understanding of the invention can be obtained by referring to the following illustrative examples which are not intended, however, to be unduly limitative of the invention.

EXAMPLE I Weight percent Binder 1 4.2

Copper flakes 16.7 Toluene 79.1

1 Blend of 50 wt. percent polyorganoslloxane (General Elem tric SE82) and 50 wt. percent polymethylmethacrylate (Rohm and Haas 456). The copper flakes were oxidized in a forced air oven for" a period of 60 minutes at different oxidation temperatures. The formulations were sprayed on aluminum alloy panels and the resulting coatings were then allowed to cure at room temperature. Thereafter, the a and e values of the coatings were determined. The oxidation temperatures and the properties of the coatings are shown below in Table I.

TABLE I Oxidation em at temp, F. are F. GsIIR Coating number:

1 Unoxldlzed 0.43 0.24 1.79 2-.. 200 0.48 0.26 1.85 3 225 0.55 0.29 1.89 250 0.61 0.83 1.85 275 0.67 0.37 1.81 300 0.67 0.39 1.72

The data in Table I show that the solar absorptance of the coatings is increased when oxidized copper flakes are used in the formulation. Furthermore, by control of oxidatio co dit o -e, t about 25 w t th p rti r formulation, it is possible to increase the or /em ratio by a substantial amount. As the oxidation temperature is inof the coatings before and after oxidation are shown below in Table III.

creased the a, value stabilizes while the e value increases, thereby decreasing the ratio. Thus, merely by controlling the oxidation conditions, it is possible to provide coatings having et /61 ratios as may he required to obtain the desired temperature distribution of a spacecraft.

EXAMPLE II Formulations were prepared according to the procedure described in Example I except that nitrocellulose was used as the binder. The composition of the formulations was as follows:

Weight percent Binder 1 7.7 Copper flakes 61.6 Lacquer thinner 2 30.7

1 Nitrocellulose.

4.5 wt. percent amyl acetate, 5 wt. percent Cellosolve acetats, 5 wt. percent toluene, and 45 wt. percent methyl isobutyl ketone, The copper flakes had an average size of about 70 microns and were oxidized for a period of 20 minutes in a forced air oven. Two coatings were prepared by spraying the formulations on aluminum alloy panels after which the coatings were cured and properties determined as inExample I. The oxidation temperatures and the properties of the coatings are shown below in Table II.

TABLE II Oxidation cm at temp., F. as 75 F. era/em 0 tin number:

EXAMPLE III Formulations were prepared according to the PIOCQQQ Ie of Example I in which unoxidized copper flakes were uses. The following are the compositions of the formulations:

Weight percent l See footnote 1 to formulation of Example I. 2 Nitrocellulose.

8 X one.

4 See footnote 2 to formulation of Example .II.

The formulations were sprayed on aluminum alley panels and thereafter cured at room temperature. The" cured coatings were then oxidized in a forced air oven ,for 20 minutes. The oxidation temperatures and the properties The foregoing data show that by oxidizing the coat ings containing unoxidized copper, the absorptance values are greatly increased with little change in emittance values. As a result very high ratios are obtained.

In the foregoing examples, solar absorptance (on) was determined by the use of a dual beam spectrophotometer with an absolute deflectance integrating sphere using a xenon energy source throughout the wavelength range. Calculation of the a, of each costing involves a proce= dure of averaging the pcrcentrcflcctance and 100% line values over a wavelength increment, then multiplying the reflectance by the percent solar intensity corre sponding to this wavelength increment as defined by F. S. Johnson, Satellite Environment Handbook," $tanford Press (1961). These values of reflected energy are summed up for each increment (a total of 25 points are used) to obtain the integrated solar reflectance, which is subtracted from 100% to obtain the solar absorptance. Magnesium oxide is used as the integrating sphere coating, but because of the sphere geometry and the dual beam of the spectrophotometer, an absolute reflectance value is obtained. I

The total normal spectral emittance is calculated from 3 to 25 microns spectral reflectance data from a double beam spectrophotometer equipped with a hohlranm heated cavity. The reflectance values are corrected for both the zero and 100% datum lines. The 100% datum line is obtained using nickel oxide as the reference. The total emittance value of a given sample is calculated from the reflectance measurements and compared to a theoretical black body at a 75 F. surface temperature.

As will be evident to those skilled in the art, various modifications of this invention can be made or followed in the light of the foregoing disclosure and discussion without departing from the spirit or scope of the invention.

We claim:

1. A. process for controlling the thermal balance of missile and spacecraft structures which comprises:

(a) oxidizing copper flakes by heating same in an oxyget -containing atmosphere at a temperature in the range of about 100 to 300 F. for a period of about 5 to 60 minutes;

(b) dispersing the oxidized copper flakes in tr solution of a curable polymeric binder in a solvent therefor so as to obtain a formulation, on a weight basis, consisting of l to 20 percent binder, '19 to 95 per= cent solvent, and 3 to percent oxidized copper flakes, the amount by weight of the flakes being greater than the amount by weight of the binder;

(c) applying a coating of the formulation of (b) to a surface of said missile or spacecraft;

(d) evaporating the solvent from the coated structures;

and?

(e) curin'g the polymeric binder with the oxidized copper flakes embedded therein to provide on the surface of said structures a coating having a solar absorptanee to infrared emittance ratio ranging from 1.80 to 2.60. 2. The process according to claim 1 in which the amount by weight of the oxidized copper flakes is greater than the amount by weight of the binder by from 5 to 8 times.

acetates.

References Cited UNITED STATES PATENTS Boebel et a1 117-33.3 Carlston et a1. 260--27 Hubbell s 260-438 Ziehl 106-290 Halberstadt 106-290 Bradshaw 106-290 5 "Selective Radioactive Coatings of Cupric Oxide.

8 OTHER REFERENCES Chemical Abstracts, vol. 57 :6057d, September 1962,

"Selective Radiation Coatings for Solar Heating.

Chemical Abstracts, vol. 57:6058e, September 1972,

WILLIAM D. MARTIN, Primary Examiner W. H. SCHMIDT, Assistant Examiner US. Cl. X-R.

117-132 B, 132 BB, 132 BS, 132 BF, 137; 244- -1 SC; 

